1. Field of the Invention
The present invention relates to a reusable or "multitime" ink sheet for use in heat transfer recording, and a production process thereof. More particularly, the present invention relates to a reusable ink sheet disposed between a printing head and printing paper in a thermal printer of a word processor, personal computer and other devices. The ink sheet according to the present invention can be advantageously used in the heat transfer recording process for an increased number of the repetition of use without deteriorating a thermal transfer capability, which relies upon a release of a portion of the ink from the sheet, and other properties thereof.
2. Description of the Related Art
Many types of reusable ink sheets have been proposed in the field of heat transfer recording. For example, Japanese Unexamined Patent Publication (Kokai) No. 57-160691 and the corresponding U.S. Pat. No. 4,661,393 to Uchiyama et al. teach an improved heat transfer recording ink sheet which comprises a substrate having formed thereon a layer of ink composition, said ink composition consisting of:
a transfer component of a solvent dye and at least one low-melting compound having a melting point in the range from 40.degree. to 100.degree. C. and containing at least one of hydroxyl and ethylene oxide; and
at least one inorganic or organic fine powder having a particle size in the range from 0.01 to 200 .mu.m, each said fine powder being insoluble and dispersible in an organic solvent.
The use of the ink sheet disclosed in the above U.S. Patent is illustrated in FIG. 1. As shown in FIG. 1, layer 3 of the ink composition is coated on one surface of the substrate 2. When heat and pressure are applied to the ink sheet 1 through a thermal printing head (not shown) in the direction of arrow, the applied heat is transmitted through the substrate 2 to reach the ink composition layer 3, whereby the ink composition distributed therein is melted and expressed therefrom. The expressed ink composition is then transferred to a receiver sheet 10 of plain recording paper to form a transferred recording 4. Thereafter, the receiver sheet 10 is peeled off from the ink sheet 1. Nevertheless, this ink sheet has a problem in that a nonuniform contact between the receiver sheet 10 and the ink composition layer 3, and accordingly a deterioration of the print quality occurs because a surface of the layer 3 is roughened, due to an unsatisfactory porous structure of the fine powder, by a repeated use of the sheet.
To solve the above-described problem, Uchiyama et al. proposed a further improved ink sheet. This ink sheet 1, as shown in FIG. 2, is characterized by comprising an ink layer 3 disposed through an interlayer 5 such as polyamide onto a substrate 2 such as a plastic sheet, for example, polyester, and containing a spongy structure of vinyl acetate resin (for example, ethylene/vinyl acetate copolymeric resin)-coated fine powders 7 such as carbon black. A transfer component 6 consisting of a black dye and a low-melting binder material such as aliphatic amide is impregnated in the spongy structure. Note, the spongy structure has a higher strength than that of the above-described porous structure of the fine powder and therefore, prevents the deterioration of the print quality. This ink sheet is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 59-165691.
Nevertheless, another problem to be solved arises with regard to the ink sheet 1 of FIG. 2, after repeated use of the sheet (see, FIG. 3), in that fine powders and a coating of ethylene/vinyl acetate surrounding the powders remain on the substrate 2 during the repeated use of the sheet; this is because they have a higher softening point than that of the low-melting material, and therefore, are not melted when the sheet is heated by the printing head, and only the transfer component 6 is melted. Accordingly, the transfer component 6 is migrated through gaps between the fine powders and portions thereof then transferred from the layer 3 to the receiver sheet 10. Although a good repeatability is obtained as a result of the above-described spongy structure, a good print density as high as that of the single use or disposable ink sheet cannot be obtained because the amount of transfer component released at each printing is relatively small.
Another type of ink sheet or reusable heat transfer ink ribbon is well-known from Japanese Unexamined Patent Publication (Kokai) No. 63-194984. The heat transfer ink ribbon of this Japanese Kokai comprises a substrate 2 and a layer 8 of molten ink applied to one surface of the substrate 2, as shown in FIG. 4, and is characterized in that this molten ink contains a specific binding agent such as ethylene/vinyl acetate copolymer, together with a colorant such as carbon black and a dispersion aid for the colorant. The binding agent is represented by the formula: ##STR1## in which R.sub.1 is a lower alkyl or hydrogen, R.sub.2 is a lower alkyl and a ratio of m/n is from 0.01 to 0.07. The described ink ribbon enables the molten ink to be completely utilized, and provide an improvement of the sharpness of the prints. As described in the working example of this Kokai, the molten ink is effectively consumed within several uses of the ribbon, but since the ink layer has a uniform composition but does not constitute a porous or spongy structure as in the above-discussed ink sheets, portions of the molten ink are not transferred from the ink layer to a surface of the printing paper. As can be seen from the cross-sectional view of FIG. 5, a substantial portion of the molten ink of the ink layer 8 is transferred to the printing paper 10 after the ribbon is once used, and thus the printing repeatability of this ribbon is not good.
In addition to the poor printing repeatability, the ink ribbon of Japanese Kokai 63-194984 has a drawback in that it is difficult to fix the ink to the paper, and therefore, the printed ink is easily removed by rubbing with the finger or by friction with other paper. The ink is easily rubbed of because the ink ribbon has a low peeling strength. The basis for this conclusion can be found in the graph of FIG. 10, showing a dependency of the peeling strength on the vinyl acetate (VA) content of the ethylene/VA copolymer described hereinafter. Namely, the m/n ratio of 0.01 to 0.07 for the above-described formula means that the VA content of the EVA copolymer is from 3 to 17.7% by weight of the copolymer. If this range of the VA content is applied to the graph of FIG. 10, it is obvious that the peeling strength of this ink ribbon is unacceptably low. Accordingly, this and other drawbacks of the above-discussed prior art ink sheets and ink ribbons must be removed to satisfy the requirements of recent, advanced heat transfer recording processes.